In the last decade a lot of effort has been put into the development of transcutaneous bone conduction implants for treating conductive hearing loss. These implants are aimed at addressing two main drawbacks of traditional percutaneous implants such as the Bone-Anchored Hearing Aid (BAHA) and the Ponto system: the need for regular care of the implant site, with increased risk of infection; and the cosmetic issues surrounding an implant sticking out of the head. The marketed transcutaneous implants are generally classified as active (Bonebridge, BCITM) or passive (Audiant, Sophono, BAHA Attract) depending upon the nature of actuation. Most of these implants (whether passive or active) transmit vibrations to skull bone through an inertial mechanism, i.e., the actuator has a vibrating mass that imparts an inertial force to the underlying skull bone.
Recently (Kotiya et al, Otol Neurotol 2016 July; 37(6):753-760, hereby incorporated by reference) a piezoelectric actuator that imparts skull bone vibration via flexural bending of the skull was proposed and demonstrated (subcutaneous piezoelectrically actuated hearing aid: SPAHA). Comparison of the bending actuator with a commercial inertial actuator showed that the bending actuator has superior performance at higher frequencies (>1800 Hz). However, the bending actuator cannot match the performance of the inertial actuators at lower frequencies since inertial actuators are designed to have resonance frequency at 700 Hz. Lacking an inertial mass, the SPAHA does not have a resonance and so has difficulty achieving large response at low frequencies.
There remains a need for a piezoelectric inertial actuator that can improve low frequency performance while maintaining superior high frequency performance in hearing aid applications.